Functionally, I'd allow one rainfall or irrigation between the Round Up and reseeding. That's to make one hundred percent sure any residue is washed into the soil, where it binds very strongly and won't have any impact on your new seeds.

Morpheuspa, that's not overkill at all. I have seen mowers track Roundup through area the next morning after Roundup was sprayed around noon the day before and the area was irrigated at night. Long story short I would wait at least a couple of days, and really you should wait at least ten days if you plan on aerifying or slit seeding the area that was sprayed. Give the Roundup time to do it's job.